Instruments provide most of the data on air quality; flora constitute a cheaper alternative, but are not without disadvantages. Plants differ in their avidities for certain naturally occurring and anthropogenic substances depending on the species, the physiological status of the specimen, growing conditions and a host of additional variables. For example, assays of contaminated foliage reflect microrelief and electrical attractions between plant surfaces and charged aerosols. Coatings on foliage and other organs continuously change as surfaces saturate independent of the mix of substances moving around them. At best, the composition of plant tissue offers a biased, time-averaged record of the chemical environment of that organism; it does not mirror the performance of the mechanical device designed to strip air of all of its contaminants.
Nevertheless, absolute and relative concentrations of certain components in ashed tissue represent useful data. For example, abnormally elevated levels of certain technological metals indicate anthropogenic contamination. Ratios of Na to Al or to some other element abundant in inland soils signal marine influence. Elements that fluctuate less in concentration relative to tissue dry weight than to percent ash, a value particularly sensitive to the thickness and composition of coatings on plant surfaces, are probably nutrients. Factor and principal component analyses more precisely define associations of elements that distinguish anthropogenic from natural sources, and differentiate constituents essential for life from those potentially harmful to organisms.
A substantial literature indicates that lichens exceed most vascular flora as worthwhile devices for air quality surveillance, and reasonably so. Most higher plants root in soil, and hence contact a source of many of the same elements that concern the environmentalist. Additionally, lichens often respond more sensitively to the gases and metals that pollute ecosystems and threaten public health. Industrial and automotive emissions already account for the collapse of formerly dense populations of lichens near many industrial and urban sources. Various conifers also exhibit exceptional susceptibility to certain air pollutants (e.g., O3), but being essentially nontransportable as adults, are useless for many applications.
Unlike most vascular flora, the epiphytes are small and easily relocated. They also grow above ground and subsist on nutrients from aerial rather than soil-based sources. Furthermore, pollutants impact some of these plants as much as they do the lichens, but with broader possibilities to quantify the effects. Stomatal conductance and associated physiology unique to vascular flora offer special opportunity for inexpensive and nondestructive diagnosis of sublethal injury and subsequent plant recovery.
Spanish moss was the first bromeliad to provide information on atmospheric chemistry, and it and several congeners continue to stock most of the surveys that employ vascular epiphytes. Wherry and Capen (1928) examined ashed shoots for signs of contamination in Florida. Martinez et al. (1971) and Robinson et al. (1973) used the same approach and bromel-iad to determine that lead from auto exhaust occurred at extraordinary concentrations in a number of roadside collections; T. usneoides also revealed Ni contamination via aerosols near a battery plant in South Carolina (Carcuccio et al. 1975). Schrimpff (1981) employed T. recurvata to map polluting metals, pesticides and aromatic hydrocarbons at two locations in Colombia, South America.
Burdens of several metals in Spanish moss increased toward a petrochemical complex in southeast Texas (Benzing 1989). Total S in the foliage of Tillandsia balbisiana, T. paucifolia and T. utriculata reached highest concentrations in collections taken closest to the urbanized coast of southeastern Florida (Benzing and Bermudes 1992). Spanish moss revealed alarming contamination from Hg vapor emanating from a gold refining operation in Brazilian Amazonia (Calasans and Malm 1994). Exposures of a few months within and near processing sheds elevated concentrations from less than one up to 60 ppm! Contaminated clothing worn by the employees who boil off metallic Hg spread the risk off-site. Calasans and Malm (1994) also placed baskets of T. usneoides inside the electrolysis chambers of a chlorine-soda facility in Brazil for 15-68 days and recovered Hg up to 13000-fold (30-35 ^m g-1 dry weight) above levels in control plants. Other samples maintained outside the factory contained 5-175 times ambient concentrations.
Experiments and horticultural practice indicate extraordinary responses among Bromeliaceae to a variety of toxic substances. Arboriculturalists formerly sprayed lead arsenate to selectively kill T. usneoides and T. recur-vata infesting shade and orchard trees in Florida. Copper salts continue to serve the same purpose. For example, two applications during one growing season of Cu(OH)2 at 35g eliminated T. recurvata on crape myrtle in southern Louisiana (Holcomb 1995). More costly Cu-based fungicides have been employed for many years to control ballmoss in Texas (Shubert 1990). Caldiz and Beltrano (1989) and Bartoli et al. (1993) achieved 100% kills of T. recurvata and T. aeranthos with little damage to Argentinian hosts using the herbicides atrazine and simazine. Experiments such as those of Benzing and Renfrow (1980) demonstrated susceptibility to overloads of several metallic micronutrients (Table 5.3).
Tolerances to corrosive gases varied according to the treatment. Tillandsia balbisiana, T. paucifolia, T. recurvata and T. utriculata survived 6-h exposures to O3 (0.15-0.45 ppm) and SO2 (0.3-1.2 ppm), applied alone and combined, in continuously stirred tank reactor exposure chambers (Benzing et al. 1992). Neither aH+ nor foliar conductance diminished during or immediately following these comparatively short nocturnal runs. Many of the test subjects flowered the following year in greenhouse culture, indicating no significant delayed injury. Remarkably low diffusive conductances even for a CAM plant probably reduced exposure of the mesophyll enough to avoid damage over such short runs. Fumigated at 3.8-4.0 ppm SO2 for several days, T. aeranthos exhibited considerable leaf necrosis (Arndt and Strehl 1989).
Shacklette and Connor (1973) and Connor and Shacklette (1984) conducted a study that exceeded all the others using bromeliads in its geographic and chemical dimensions. Briefly, factor analysis identified three series of elements that accounted for almost three-quarters of the variation in log concentrations in the ash of 123 samples of Spanish moss collected across the southeastern United States. Aluminum, Ba, Ca, Co, Ga, Fe, Mn, Ti, Yb and Zr constituted a pedological association present in samples from all sites. Three more elements (Ca, Na and Sr) that occur abundantly in less widely distributed substrates were associated with, but somewhat distinct from, the other 10. Inconsistent proportions between the concentrations of members of the first series of 14 elements and total plant ash suggested enrichments by wind-deposited particles. Plant absorption contributed far less.
Distributions of several of the elements in this first series among the samples indicated different terrestrial sources. Limestone dust from roadbed fill and quarrying operations probably produced the frequent, proportionally high values for Ca and Sr relative to Al. Collections with the highest Na:Al ratios usually came from Florida and the Gulf and Atlantic coast sites (Shacklette and Connor 1973) to the west and north, indicating strong marine influences. Presumably coatings on leaves near the ocean contained more sea salt and marine limestone dust, whereas materials derived from terrestrial soils predominated farther inland.
Nine elements (B, Cr, Cd, Cu, Li, Ni, Pb, V and Zn) formed a second series made up principally of the technological metals. Generally low logarithmic correlations of these constituents with log Al in ash suggested that sources other than soil accounted for their occurrence in the samples. Lead concentrations peaked near heavily traveled highways, as noted in some of the other studies with T. usneoides (e.g., Martinez et al. 1971). Occurrences of Zn, Cu and Cd in the same vector fan of the Connor and Shacklette varimax model further indicated that these plant constituents originated from vehicles, most likely from lubricants, tires and abraded body parts, if not exhaust. The same explanation probably applied to Cr and Ni.
Boron, Li and V emerged at the other end of the technological metals vector fan. Residual oil used for heating and power generation, in addition to some soils, represent potentially major sources of V in the southeastern United States (Schroeder 1970; Zoller et al. 1973). Glass manufacture employs considerable Ba and Li, and Cd, Cr, Ni, V and Zn indicate other industrial processes. Coal-burning volatilizes many metals. Connor and Shacklette concluded that, unlike the soil-element association, anthropogenic aerosols rather than absorbed natural particulates accounted for the extraordinary accumulations of technological metals.
A geographic dimension further differentiated the series one and series two metals. Substratum-related elements exhibited broad regional (e.g., landward vs. coastal) trends in their proportional concentrations in Spanish moss, while the technological metals showed more localized occurrences reflecting nearby discrete sources. Sheline and Winchester (1976) also identified characteristic combinations of elements in samples of Spanish moss in northern Florida, which they considered consistent with plant uptake of aerosol particles.
Magnesium, P and K formed a third varimax series in Connor and Shacklette's study, distinguished from the others by pattern of occurrence and status as plant macronutrients. Log concentrations of all three elements varied independently of the other 23 surveyed and percent ash, reputedly because the bromeliad controlled their accumulation according to need.
Two inherent characteristics probably account for the remarkable affinities Type Five bromeliads exhibit for diverse substances: dependence on the atmosphere rather than soil for nutrients, and a relatively nondis-criminating organ (the foliar trichome) for absorption. Capacity to scavenge often scarce ions is essential for these plants. However, the same mechanisms apparently promote toxic accumulations of required (e.g., Cu) and other substances if supplies become too enriched. Consequently, olig-otrophic members of Tillandsia, probably more than most vascular flora, offer exceptional opportunity to monitor air quality. However, as slow-growing CAM plants, these bromeliads do not match the sensitivity of much other vegetation (e.g., certain crops) to chronic exposures to several common corrosive gases.
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